1. Field of the Invention
The present invention relates to a piezoelectric actuator used for, for example, a positioning device of a precision apparatus, and is applied to, for example, a scanner for a scanning probe microscope.
2. Description of the Related Art
Up to now, a piezoelectric actuator has been used for a precision positioning device with precision of the order of sub-nanometers to several hundred micrometers in various precision apparatuses including measurement apparatuses.
Hereinafter, the positioning device using the piezoelectric actuator is described by way of example of a scanning probe microscope (see JP 09-089913 A).
FIG. 22 is a structural diagram illustrating a conventional scanning probe microscope. The conventional scanning probe microscope includes a cantilever 214 having a probe 213 at the tip, a sample holder 211 which is opposed to the probe 213 and used to place a sample 212, a triaxial fine adjustment mechanism 215, and a displacement detection mechanism 219 for detecting bending of the cantilever 214. The triaxial fine adjustment mechanism 215 includes a lateral fine adjustment mechanism for moving the probe 213 in in-plane directions of the sample and a vertical fine adjustment mechanism for moving the probe 213 in a direction perpendicular to a surface of the sample.
In the conventional technique illustrated in FIG. 22, a piezoelectric actuator including a cylindrical piezoelectric element is used as the triaxial fine adjustment mechanism 215. The cylindrical piezoelectric element is polarized such that inner crystals are uniformly aligned in a direction orthogonal to the center axis of the cylinder between an inner surface and an outer surface of the cylindrical piezoelectric element. A single common electrode 232 is formed on the inner surface of the cylindrical piezoelectric element, and a band-shaped electrode portion 235 and four-part electrode portions 233 and 234 are formed on the outer surface thereof. The band-shaped electrode portion 235 is provided along the circumference of the cylindrical piezoelectric element. The four-part electrode portions 233 and 234 are obtained by dividing the cylindrical piezoelectric element into four around the circumference and provided in a direction parallel to the center axis. When the side on which the band-shaped electrode portion 235 is provided is assumed to be a tip end and the side on which the four-part electrode portions 233 and 234 are provided is assumed to be a terminal end, the cantilever 214 is attached to the tip end, and the terminal end is fixed to a base (not shown).
In the cylindrical piezoelectric actuator, the four-part electrode portions 233 and 234 serve as the lateral fine adjustment mechanism and the band-shaped electrode portion 235 serves as the vertical fine adjustment mechanism. When the cylindrical piezoelectric actuator is to be driven, the common electrode 232 formed on the inner surface is connected with a ground potential terminal, and voltages having reverse phases are applied to two electrodes which are opposed to each other about the center axis of the four-part electrode portions 233 and 234. At this time, one of the electrodes extends in the direction parallel to the center axis and the other electrode contracts. As a result, bending occurs in the cylindrical piezoelectric element, whereby the tip end performs arc motion. The amount of movement during the arc motion is considerably small, and hence the probe 213 can be moved substantially parallel to the in-plane of the sample 212. The other two electrodes opposed to each other are also operated in the same manner. Therefore, the probe 213 can be two-dimensionally moved within the in-plane of the sample 212.
When a voltage is applied to the band-shaped electrode portion 235 formed on the outer surface, strain occurs in the diameter direction. As a result, strain also occurs in the direction parallel to the center axis. Therefore, the probe 213 can be moved in a direction orthogonal to the sample 212.
An optical lever method is normally used for the displacement detection mechanism for the cantilever 214. The displacement detection mechanism 219 includes a semiconductor laser 216, a condenser lens 217, and a photo detector 218. Light from the semiconductor laser 216 is focused on a rear surface of the cantilever 214 by the condenser lens 217. The light reflected on the rear surface of the cantilever 214 is detected by the photo detector 218. When bending occurs in the cantilever 214, a position of a spot on the photo detector 218 changes. Therefore, when the amount of change is detected, the bending of the cantilever 214 can be detected.
When the probe 213 is brought close to the sample 212 in the scanning probe microscope having the structure described above, the cantilever 214 is bent by the action of the interatomic force or contact force. In this case, the amount of bending depends on a distance between the probe 213 and the sample 212. Therefore, the amount of bending is detected by the displacement detection mechanism 219 for the cantilever 214. The vertical fine adjustment mechanism is operated by a control circuit 221 such that the amount of bending becomes constant. While feedback control is performed such that the distance between the probe 213 and the sample 212 becomes constant, raster scanning is performed using the lateral fine adjustment mechanism by a scanning circuit 222. Thus, an unevenness image on the surface of the sample can be measured. In addition to the measurement method using the contact system for detecting the static bending of the cantilever 214, there is a measurement method using a vibration system, in which the distance between the probe 213 and the sample 212 is controlled based on the amount of change in amplitude, phase, or frequency which is caused by the action of the interatomic force or intermittent contact force while the cantilever 214 is vibrated at the vicinity of the resonance frequency.
The triaxial fine adjustment mechanism 215 used as the positioning device of the scanning probe microscope includes the piezoelectric element, and hence a hysteresis or creep occurs. The hysteresis is a phenomenon in which, when a voltage is applied to the piezoelectric element, a displacement corresponding to the voltage does not become completely linear but performs such behavior as approximated by a quadratic curve. The creep is a phenomenon in which, when a voltage is applied to the piezoelectric element, the displacement does not immediately reach the target amount of movement but gradually and finely changes with time.
When the hysteresis or creep occurs, it is difficult to perform accurate positioning. Therefore, there is a system for detecting a displacement of a positioning device by a displacement detection device for detecting the displacement of the piezoelectric element as a more precise positioning means, to compensate for hysteresis or creep.
Various systems such as an optical sensor, a capacitance sensor, and a magnetic sensor are used for the displacement detection device for detecting the displacement of the piezoelectric element. A detection method using a strain gauge is effective as a method which requires a minimum space and is low cost and convenient.
FIG. 23 illustrates the piezoelectric actuator provided with a displacement meter for detecting the displacement of the triaxial fine adjustment mechanism 215 of the scanning probe microscope by strain gauges according to the conventional technique. In the conventional technique, strain gauges 201a, 201b, 202a, and 202b are bonded to the respective electrodes of the four-part electrode portions 233 and 234 formed on the outer surface of the cylindrical piezoelectric element. Two strain gauges 203a and 203b are bonded to the band-shaped electrode portion 235 in parallel to the center axis. Each of the strain gauges is a normally available strain gauge, and is bonded in a direction in which large output is obtained when strain occurs in a direction parallel to the center axis of the cylindrical piezoelectric element. In the normal strain gauge, an insulating material such as a polyimide resin, paper, a phenol resin, an epoxy resin, or a phenol/epoxy-mixed resin is used for a base material. A metal material such as a copper-nickel alloy or a nichrome-based alloy or a resistor made of a semiconductor such as single-crystal silicon is provided on the base material and electrically connected with an external detection device through an electrode pattern which is formed on the base material and made of, for example, nickel.
A bridge circuit as illustrated in FIG. 24 is incorporated into the lateral fine adjustment mechanism. The bridge circuit includes a pair of strain gauges 201a and 201b or 202a and 202b bonded to two opposed electrodes 233 and 234 and two fixed resistors 241 and 242. A bridge voltage e0 is applied to the bridge circuit to measure an output voltage e1. When strain occurs in the piezoelectric element, resistance values of the strain gauges 201a, 201b, 202a, and 202b are changed to change a value of the output voltage e1. When the output voltage e1 is detected, the amount of strain of the piezoelectric element can be measured. In the conventional technique, the pair of strain gauges 201a and 201b and the pair of strain gauges 202a and 202b for the respective axes are each bonded to the electrodes 233 and 234 opposed about the center axis. Therefore, when the lateral fine adjustment mechanism is bent around the center axis, respective strain directions become reverse to each other, and hence signs of detection signals of the pair of strain gauges also become reverse to each other. Thus, an output voltage two times larger than in a case where a strain gauge is bonded to only a single electrode can be obtained, thereby increasing a signal strength with respect to noise. A variation in resistance value due to a temperature change is canceled as temperature compensation.
In the case of the vertical fine adjustment mechanism, a bridge circuit as illustrated in FIG. 25 is incorporated thereinto. The bridge circuit includes two strain gauges 203a and 203b and two fixed resistors 241 and 242. The bridge voltage e0 is applied to the bridge circuit to measure the output voltage e1. When strain occurs in the piezoelectric element, resistance values of the strain gauges are changed to change the value of the output voltage e1. When the output voltage e1 is detected, the amount of strain of the piezoelectric element can be measured. Even in this case, an output voltage two times larger than in the case of the single strain gauge can be obtained. Note that a variation in resistance value due to a temperature change is not canceled for compensation in the bridge circuit.
For the output of the strain gauge, the output voltage e1 and the displacement are calibrated based on data obtained when a calibration sample is measured by another displacement meter whose displacement is calibrated in advance or the scanning probe microscope using the triaxial fine adjustment mechanism 215. Therefore, the amount of displacement can be measured from the obtained output voltage e1.
As described above, the feedback control is performed based on the displacement information obtained from the output voltage of the strain gauges continuously, to linearly operate the triaxial fine adjustment mechanism according to an applied voltage. The scanning probe microscope is not necessarily linearly operated in the vertical direction according to the voltage. There is also a case where height information obtained from the output signal of the strain gauges is displayed without any processing.
However, in the conventional piezoelectric actuator having the structure as described above, the strain gauges are directly bonded through intermediation of an insulator to surfaces of the electrodes which are applied with the drive voltage for the piezoelectric element. Therefore, the insulator is sandwiched by the surface of the electrode of the piezoelectric element and the resistor of the strain gauge or a resistor connection electrode connected with the resistor. The base material serves as a dielectric, and hence the strain gauge bonding portions act like capacitors, thereby generating a capacitance component. Thus, the detection signal of the strain gauges cannot be accurately measured because of the influence of the capacitance component.
A high voltage for driving the piezoelectric element is applied between the surfaces of the electrodes of the piezoelectric element. Therefore, a lead wire connected with the strain gauge may be brought in contact with an electrode associated with the piezoelectric element, or the electrode or the resistor of the strain gauge may be connected with the electrode of the piezoelectric element through an inner portion of the base material of the strain gauge because of a deterioration with time. Then, the high voltage is applied to the strain gauge or the displacement detection device connected with the strain gauge, with the result that the strain gauge or the displacement detection device may be broken. In addition, when an insulation resistance of the base material is low, there is a case where the detection signal of the strain gauge cannot be accurately measured because of a leak current.